The present invention relates to power drive circuits using active semiconductor devices.
Background: "MOS" Transistors
One of the basic types of transistors is the insulated-gate field-effect transistor, which is commonly also referred to as a "MOS" transistor or "MOSFET". In any MOS transistor, the voltage on the "gate" terminal controls the density of majority carriers (electrons or holes) in a semiconducting "channel" region which is part of the conduction path. By controlling the number of majority carriers in the channel, the flow of current from the "source" through the channel and to the "drain" is controlled. Thus a MOS transistor operates as a switch which is controlled (at least partially) by the voltage on its gate.
A "MOSFET" may be configured to be a lateral, vertical, or grooved vertical structure. There are four regions in a MOSFET: source, drain, body, and gate. The source is a region where carriers originate when current is flowing in the transistor in normal operating conditions. The drain is a region where the carriers terminate when current is flowing in the transistor in normal operating conditions. The body is a region whose surface is inverted to form a "channel" when carriers flow in the transistor in normal operating conditions. The gate is an electrode that covers the surface of the body region between the source and the drain regions, and controls the carrier flow between the source and the drain of the transistor in normal operating conditions.
Background: N-Channel versus P-Channel
MOSFETs can be either n-channel devices (in which the majority carriers are electrons) or p-channel devices (in which the majority carriers are holes). The relative performance of equivalent n-channel and p-channel MOSFETs is determined primarily by the "mobility" of the electrons and holes in the inverted "channel" region at the surface of the body region. Electrons generally have a higher mobility, so the resistance of an n-channel MOSFET is lower than a p-channel MOSFET with the same geometry. (For example, in silicon at room temperature the electron mobility is about 2 to 3 times the hole mobility, and the ratio of mobilities is even larger in gallium arsenide; but in some semiconductors, such as germanium or gallium phosphide, the hole mobility is more nearly equal to the electron mobility.
N-channel transistors also differ from P-channel transistors in some other ways which can be important for power devices. For example, the transient behavior at turn-on and turn-off can be slightly different. This difference can be important for power devices, where mismatch during turn-on (or during turn-off of an inductive load) may lead to disastrous side-effects. In any case, N-channel and P-channel MOS-gated devices rarely have the same switching characteristics for the same geometry.
Background: Overdrive
An N-channel MOS transistor begins to turn "on" when the gate voltage (measured with respect to source voltage) exceeds the threshold voltage (V.sub.T). The amount by which the gate voltage exceeds the threshold voltage will be referred to as the "overdrive" voltage. As the overdrive voltage increases, the transistor passes more and more current, up to a "saturation" current value. If the gate voltage is increased further, the current increases much more slowly. However, with power transistors it is common to increase the gate voltage beyond the point where saturation occurs. This gate voltage increase is used to decrease the on-resistance of the transistor as much as possible.
Background: DMOS
A "DMOS" transistor is a particular type of MOS transistor which is commonly used for high-voltage applications. A DMOS device uses a short channel which is defined by differential diffusion, in combination with a drift region which is interposed between the channel and the drain. (The drift region helps to stand off high source-drain voltages when the channel has not been turned "on.") For high-power applications, one commonly used form is a vertical DMOS, as shown in FIG. 1. In the solid structure shown, majority carriers (electrons in this example) flow laterally from source regions 12 through channel regions 14 into drift regions 16, within which the carriers pass vertically to drain 18 on the backside of the semiconductor chip.
MOS-gated devices with their high input impedance were viewed as nearly ideal switches. However, it was the use of the DMOS structure to fabricate power transistors that began a period of significant development in MOS-gated power devices. This evolution is covered in several books and articles, such as "Trends in Power Semiconductor Devices" by B. Jayant Baliga (43 IEEE TRANSACTIONS ON ELECTRON DEVICES 1717 -1731, October 1996); Chapter 1 titled "Power Semiconductor Devices for Variable Frequency Drives" written by B. Jayant Baliga in POWER ELECTRONICS AND VARIABLE FREQUENCY DRIVES (ed. B. Bose 1996); and POWER SEMICONDUCTOR DEVICES by B. Jayant Baliga (PWS Publishing Company, 1996). Other, newer MOS-gated power devices based on DMOS technology such as the insulated-gate thyristors or IGTH (J.S.Ajit and D.M. Kinzer, "1200V, 150A Insulated-Gate Thyristors," PROCEEDINGS OF 1995 INTERNATIONAL SYMPOSIUM ON POWER SEMICONDUCTOR DEVICES & I.C.s.) and the base open/short-controlled thyristor or BOSCHT have also been developed. (J.S. Ajit, "A New Three-Terminal Thyristor-Based High-Power Switching Configuration with High-Voltage Current Saturation," 18 IEEE ELECTRON DEVICE LETTERS, July 1997.)
Double-diffused MOS or DMOS transistors are MOSFETs with a structure that eliminates many of the on-resistance and voltage limitations of conventional MOSFETs. DMOS transistors use the difference in the diffusion of sequentially introduced body and source impurities from a common edge to determine the channel length. Control of both the channel length and the peak dopant concentration in the body are obtained, as in double-diffused bipolar technology, by control of the amount of dopant introduced at the body and source doping step, and by the subsequent diffusion cycle. A DMOS transistor differs from a conventional MOS transistor in two distinct ways:
1. Since the channel Length L is determined by the difference between two sequential diffusions moving in the same direction from a common point of origin, L can be reproducibly controlled to values in the 0.5-to 2-micrometer range. PA0 2. The body region is more heavily doped than the N-drain region, resulting in a junction that depletes further into the drain region than into the body region when a reverse bias is placed across the drain-to-body junction. This difference allows significantly higher voltages to be placed across the body-to-drain junction without markedly affecting the electrical channel length of the transistor.
These two differences in device structure result in MOS transistors with both a short channel length and the ability to withstand high drain-to-source voltages due to the separation of the active or channel region of the device from the region of the device that sustains the drain-to-source voltage. The DMOS transistor structure is somewhat analogous to that used in double-diffused bipolar transistors for many years. (In bipolar transistors, a narrow, moderately doped base region controls device electrical characteristics. A lightly doped N-collector region is used to support applied potentials.)
A typical circuit application of DMOS is a half-bridge or "phase leg", shown in FIG. 2. The half-bridge uses two identical N-channel MOS transistors Q.sub.1E and Q.sub.2E in series, but with at most one of the transistors turned "on" at a time. The transistor between the more positive power supply terminal and the load is referred to as the "high-side" device.
Background: Depletion-Mode MOS Devices
An N-channel MOS transistor begins to conduct when its gate voltage (measured with respect to its source voltage) rises above a certain "threshold" voltage (which is determined by the composition, dopings, and dimensions of any particular device). If the threshold voltage is positive (as is normal), the transistor is normally "off", and is referred to as an "enhancement-mode" device. Most power MOS transistors have generally been enhancement mode (normally "off") devices.
However, if the threshold voltage of an N-channel transistor is below zero, the device is referred to as a "depletion-mode" device. In this case the transistor is at least partially "on" with a gate voltage of zero, and the gate voltage must be driven below zero to fully turn the transistor "off." An advantage of N-channel devices is that it is generally easier to generate a constant negative voltage than to generate a positive voltage that is referenced to the voltage present on the source (or equivalent terminal) of a high-side drive device.
In VLSI devices the threshold voltage is typically controlled by an ion implantation into the semiconductor material of the channel. However, researchers have demonstrated N-channel depletion-mode power transistors, in which the threshold voltage is shifted by an implant into the gate insulator. Even though the gate insulators in power MOS transistors are typically much thicker than in VLSI transistors, implantation into the gate insulator does work well in power MOS devices. See e.g. UK Patent 2,028,528, U.S. Pat. Nos. 3,328,210, 4,774,196, 4,799,100, 4,868,537, 4,958,204, 4,978,631, and U.S. patent application Ser. No. 06/771,444 "Method for Shifting the Threshold Voltage of DMOS Transistors (referenced in issued U.S. Pat. No. 4,978,631)" all of which are hereby incorporated by reference.
The degree of depletion obtained by implantation into the gate insulator is a function of the distance the permanently charged ions are above the gate dielectric-semiconductor interface and the original threshold voltage of the transistor. In an N-channel MOSFET with a gate dielectric consisting of 0.1.times.10.sup.-6 meter of silicon dioxide, a cesium implant of 3.5.times.10.sup.12 /cm.sup.2 is sufficient to move the threshold voltage of a 600V MOSFET from plus 2-4 Volts to minus 6-8 Volts. This change in V.sub.T of approximately 10 Volts produces a DMOS transistor that is turned "on" by the permanently ionized impurities in the gate by 6-8 Volts. For the purposes of this disclosure, a transistor that is turned "on" using permanently charged ions by a voltage that is more than twice the unadjusted V.sub.T value will be referred to as being in "deep depletion."
Power DMOS transistors, whether discrete devices, incorporated in integrated circuits, or merged into discrete devices such as IGBTs, MCTs, IGTHs, have almost always been enhancement mode or normally "off" devices. In applications requiring a power switch in series with the load, but in a "low-side" configuration, N-channel power devices are suitable. However, several common power switching configurations (e.g. the high side switch, the half-bridge, and the H-bridge) place different requirements on the switch(es). Looking first at a high-side switch as shown in FIG. 3, an N-channel power MOS transistor (or alternatively a MOS-gated power device with a merged N-channel MOSFET) is the power device Q.sub.NE. (The use of a P-channel power MOS transistor, or alternatively a MOS-gated power device with a merged P-channel MOSFET, is also possible, but the difference in device characteristics that result from the difference between the mobility of conduction electrons and holes, and/or the resulting cost premium that must be paid for P-channel or P-channel based MOS-gated power devices, often precludes their use.) On the other hand, the use of an N-channel MOS-gated power device (which includes N-channel MOSFETs as well as merged devices based on N-channel MOSFETs) in high-side switch configurations requires a gate voltage that is referenced to the voltage present on the source (or equivalent) terminal S (FIG. 3). If the voltage on the drain (or equivalent) terminal D is the highest voltage in the circuit, a drive voltage that is greater than the voltage on the drain (or equivalent) terminal is almost always needed to turn the switch "on". In a half-bridge, the requirements for driving a "high side" N-channel MOS-gated power device are extremely similar to those in a high side switch, as are the issues concerning P-channel devices. In addition, a P-channel device with characteristics that match those of the low-side N-channel device may not be available.
Background: "MOS-Gated" Power Devices
A wide variety of power semiconductor devices have been proposed. Many make use of voltage-controlled conduction in a channel region in combination with other mechanisms, and such devices, as well as MOS transistors, are collectively referred to herein as "MOS-gated" devices. One very simple example is the "IGBT", in which minority carriers are injected at the drain end of the channel to increase total current for a given flow of electrons. However, there are many other MOS-gated power devices (including MCTs, IGTHs, and many others), as described e.g. in B. J. Baliga, POWER SEMICONDUCTOR DEVICES (1995), which is hereby incorporated by reference.
Background: High-Side Switching
A high-side switch, where the switch is between the more positive power supply terminal and the load, is required in many applications. One such application is motor switching. From a circuit design perspective, a P-channel device would be easiest to drive and a logical choice for high-side switches. However, N-channel devices are generally used because they have better transconductance and therefore are smaller in area than P-channel devices. Unfortunately, N-channel devices, when operated in the common-drain configuration, may require a gate voltage that is greater than the supply voltage. If the voltage on the drain (or equivalent) terminal is the highest voltage in the circuit, a drive voltage that is greater than the voltage on the drain (or equivalent) terminal is usually needed to turn the switch "on. " This requires use of a charge pump, additional voltage supply, bootstrap techniques, or other scheme to create the necessary voltage. FIG. 3 shows an example of prior art high-side switching with an N-channel enhancement device Q.sub.NE and a charge pump CP. The charge pump CP is required to create a gate drive voltage for Q.sub.NE that is higher than the +VCC supplied to the drain of Q.sub.NE.
Various drive techniques for high side n-channel MOS-gated power devices have been developed. These techniques include charge pumps, independent high-side power supplies, and bootstrap techniques (see "Drive Techniques for High Side N-channel MOSFETs" by Warren Schultz, Power Conversion & Intelligent Motion, June, 1987, which is hereby incorporated by reference.) The document "INT 978," hereby incorporated by reference, available from the International Rectifier web site discusses the gate drive requirements of high-sided MOS-gated devices. Table 1 in INT 978 discusses the following five gate drive methods: Floating Gate Drive Supply, Pulse Transformer, Charge Pump, Bootstrap, and Carrier Drive.
Background: H-Bridge (Full-Bridge) Switches
An H-bridge, also known as a full-bridge, consists of two phase-legs, A and B, with a load between the legs, as shown in FIGS. 4A and 4B. The input is usually a fixed DC voltage. The output is a voltage that can be controlled in magnitude and polarity. Typically, each leg consists of two switches and their antiparallel diodes. The two switches in each leg are switched in such a manner that when one of them is "on," the other is "off." In practice, both switches generally are "off" for a short interval, known as the blanking time. Otherwise, inadvertent on-time overlap in a phase-leg's high-side and low-side switches might result in an effective short circuit that can allow "shoot-through" current that may damage the switches.
If the switches in each leg are driven so that both the switches are not "off" simultaneously, then current will flow to the load continuously. The output voltage of each leg depends only on its input voltage and switch duty cycle. Therefore, with a constant input voltage, the voltage to the load is dictated by the status of the phase-leg switches.
Phase Leg (Half-Bridge) with Depletion-Mode Device
The presently preferred embodiment incorporates a depletion-mode n-channel MOS-gated device as a high-side switch in a phase leg. The depletion-mode N-channel MOS-gated switch has its threshold voltage shifted so it is fully "on" with a gate-to-source voltage of zero volts. The use of normally "on" MOS-gated devices allows the use of a negative switching voltage that is referenced to the voltage on the source of the low-side switch.
To keep the bottom switch "off" during start-up or power interruptions, two techniques are: (1) place a resistor between the gate and the source of the bottom device; or (2) as seen in one embodiment of this invention shown in FIG. 5, place a low voltage normally "on" device such as a JFET or depletion mode MOSFET Q.sub.3D between the bottom switch's gate and source, with the gate tied to the negative reference voltage. The device is "on" when no power is present, keeping the gate and source tied together.
During normal operation, the timing of the drive signals to the top and bottom devices guarantees that no shoot through can occur.
The MOS-gated high-side switch described herein can be used in any high-side switch application. Typical applications include high-side switches in automotive and similar applications. The leg of an H-bridge described herein can be used in any conventional H-bridge application which includes motor drives, power supplies, and driving lamps in lighting applications.
The disclosed innovations, in various embodiments, provide one or more of at least the following advantages:
high-side semiconductor switching without driver charge pumps, independent high-side power supplies, or bootstrap techniques PA1 phase-leg switch elements which have substantially identical diffusion structures and geometries and require little or no tweaking for matching, resulting in essentially identical electrical characteristics (except for V.sub.T) PA1 all switches can be integrated onto an IC if the voltage capability of the IC technology exceeds the peak voltage PA1 negative voltage control of the high-side switch PA1 active control of the high-side and low-side switch PA1 protection against shoot through during power-up PA1 all switches may be switched directly, allowing controllers or microprocessors to determine the states of both the high-side and low-side switches